Amino acid metabolism
58 flashcards covering Amino acid metabolism for the MCAT Biology & Biochemistry section.
Amino acid metabolism encompasses the body's processes for handling amino acids, the building blocks of proteins. These processes include breaking down amino acids for energy through catabolism, synthesizing new ones via anabolism, and converting them into other molecules like nucleotides or neurotransmitters. This metabolism is essential for maintaining nitrogen balance, detoxifying waste products, and supporting overall cellular function, making it a key aspect of biochemistry.
On the MCAT, amino acid metabolism frequently appears in biology and biochemistry questions that test knowledge of pathways like transamination, deamination, and the urea cycle, often integrated with enzyme regulation or clinical disorders such as phenylketonuria. Common traps include confusing these pathways with carbohydrate metabolism or overlooking the role of cofactors; focus on identifying essential versus non-essential amino acids and their interconnections with energy production. Practice sketching key pathways to reinforce your grasp.
Terms (58)
- 01
Amino acid
An organic molecule containing an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain, serving as the building blocks of proteins and participating in various metabolic processes.
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Essential amino acids
Nine amino acids that humans cannot synthesize and must obtain from the diet, including histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
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Non-essential amino acids
Amino acids that the human body can synthesize from other compounds, such as alanine, asparagine, aspartate, and glutamate, though their production may depend on adequate nutrition.
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Zwitterion
A molecule that has both a positive and a negative charge, such as an amino acid at its isoelectric point, where the amino group is protonated and the carboxyl group is deprotonated.
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Peptide bond
A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water and linking amino acids into peptides and proteins.
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Transamination
A reaction that transfers an amino group from an amino acid to an alpha-keto acid, typically catalyzed by transaminases, allowing amino acids to be interconverted for metabolism.
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Deamination
The removal of an amino group from an amino acid, producing ammonia and a keto acid, which is a key step in amino acid catabolism to generate energy or intermediates.
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Oxidative deamination
A process primarily involving glutamate, where the amino group is removed and oxidized to ammonia by enzymes like glutamate dehydrogenase, linking to the electron transport chain.
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Glutamate dehydrogenase
An enzyme that catalyzes the reversible oxidative deamination of glutamate to alpha-ketoglutarate and ammonia, playing a central role in nitrogen metabolism and energy production.
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Urea cycle
A metabolic pathway in the liver that converts ammonia, a toxic byproduct of amino acid metabolism, into urea for excretion in urine, involving five main enzymatic steps.
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Ornithine
A non-proteinogenic amino acid that serves as a substrate in the urea cycle, accepting a carbamoyl group to form citrulline and regenerating at the end of the cycle.
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Citrulline
An amino acid formed in the urea cycle from ornithine and carbamoyl phosphate, which then combines with aspartate to produce argininosuccinate.
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Argininosuccinate
An intermediate in the urea cycle formed from citrulline and aspartate, which is cleaved to produce arginine and fumarate.
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Fumarate
A four-carbon intermediate produced in the urea cycle from argininosuccinate, which enters the citric acid cycle to be further metabolized for energy.
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Arginine
A semi-essential amino acid that is the immediate precursor to urea in the urea cycle, formed from argininosuccinate and hydrolyzed to produce ornithine and urea.
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Glucogenic amino acids
Amino acids that can be converted into glucose through gluconeogenesis, such as alanine and aspartate, by providing carbon skeletons that enter the glycolytic pathway.
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Ketogenic amino acids
Amino acids that can be converted into ketone bodies, such as leucine and lysine, which are primarily degraded to acetyl-CoA or acetoacetyl-CoA.
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Alanine cycle
A process where alanine carries amino groups from muscles to the liver, where it is transaminated to pyruvate and used for gluconeogenesis, helping maintain blood glucose.
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Pyruvate from alanine
The product of transamination of alanine, which can be converted to glucose in the liver or oxidized in the mitochondria, linking amino acid metabolism to glycolysis.
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Oxaloacetate from aspartate
The keto acid formed when aspartate undergoes transamination, serving as a key intermediate in the citric acid cycle and gluconeogenesis.
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Alpha-ketoglutarate from glutamate
The product of deamination of glutamate, which is a citric acid cycle intermediate and can accept amino groups in transamination reactions.
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Phenylalanine metabolism
The pathway where phenylalanine is hydroxylated to tyrosine by phenylalanine hydroxylase, and defects can lead to phenylketonuria, causing intellectual disability if untreated.
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Tyrosine metabolism
The process converting tyrosine to catecholamines like dopamine and to melanin, with intermediates used in thyroid hormone synthesis and potential accumulation in disorders.
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Tryptophan metabolism
A pathway that converts tryptophan to niacin, serotonin, and melatonin, involving steps like hydroxylation and requiring vitamin B6 as a cofactor.
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Histidine metabolism
The degradation of histidine to formiminoglutamate, which enters the one-carbon pool, and its essentiality in children due to growth demands.
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Cysteine metabolism
The synthesis of cysteine from methionine and serine, involving steps like transsulfuration, and its role in forming glutathione and other sulfur-containing compounds.
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Methionine metabolism
The conversion of methionine to S-adenosylmethionine for methylation reactions, and its role in the one-carbon metabolism cycle via homocysteine.
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Folate in one-carbon metabolism
A B vitamin that carries one-carbon units in amino acid metabolism, such as in the synthesis of methionine from homocysteine, and is crucial for nucleotide production.
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Vitamin B12
A cofactor required for the conversion of homocysteine to methionine and for the rearrangement in methylmalonyl-CoA mutase, linking to fatty acid metabolism.
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Homocysteine
An intermediate in methionine metabolism that can be remethylated to methionine or converted to cysteine, with elevated levels linked to cardiovascular disease.
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S-adenosylmethionine
A activated form of methionine that donates methyl groups in various reactions, including neurotransmitter synthesis and DNA methylation.
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Transmethylation reactions
Processes where methyl groups are transferred from S-adenosylmethionine to acceptors like DNA, proteins, or neurotransmitters, regulating gene expression and signaling.
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Urea
The main nitrogenous waste product formed in the urea cycle from ammonia and carbon dioxide, which is excreted by the kidneys to maintain nitrogen balance.
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Nitrogen balance
The state where nitrogen intake from proteins equals nitrogen loss, important for assessing protein metabolism and nutritional status in the body.
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Protein turnover
The continuous process of protein synthesis and degradation, where amino acids are recycled or used for energy, regulated by nutritional and hormonal factors.
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Amino acid catabolism
The breakdown of amino acids to release energy, involving deamination and the entry of carbon skeletons into metabolic pathways like the citric acid cycle.
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Anaplerotic reactions
Reactions that replenish intermediates in the citric acid cycle, such as the conversion of amino acids like aspartate to oxaloacetate.
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Branched-chain amino acids
Leucine, isoleucine, and valine, which are essential and primarily metabolized in muscles, with defects leading to maple syrup urine disease.
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Maple syrup urine disease
A disorder due to deficiency in branched-chain alpha-keto acid dehydrogenase, causing buildup of branched-chain amino acids and a characteristic odor in urine.
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Phenylketonuria
An inherited disorder from phenylalanine hydroxylase deficiency, leading to phenylalanine accumulation and potential neurological damage if phenylalanine intake is not restricted.
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Alkaptonuria
A rare disorder where homogentisic acid accumulates due to deficiency in homogentisate oxidase, causing dark urine and connective tissue pigmentation.
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Albinism
A condition from defects in tyrosine metabolism enzymes, resulting in reduced melanin production and increased risk of skin cancer from UV exposure.
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Regulation of amino acid metabolism
Control mechanisms including enzyme induction, allosteric regulation, and hormonal influences like insulin promoting amino acid uptake for protein synthesis.
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Amino acid transport
The movement of amino acids across cell membranes via carriers, such as sodium-dependent transporters in the intestine and kidneys for absorption and reabsorption.
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Active transport of amino acids
Energy-dependent processes using ATP or sodium gradients to move amino acids against their concentration gradient, essential for nutrient uptake in cells.
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Sodium-dependent transporters
Proteins that couple amino acid uptake with sodium ion influx, facilitating absorption in the gut and reabsorption in the renal tubules.
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Strategy for remembering essential amino acids
Use mnemonics like 'PVT TIM HALL' to recall the nine essential amino acids: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, and Lysine.
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Common trap: Confusing glucogenic and ketogenic
Many amino acids are both, but remember that glucogenic ones yield pyruvate or cycle intermediates, while ketogenic ones produce acetyl-CoA, and some like leucine are only ketogenic.
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Worked example: Transamination of alanine
In transamination, alanine donates its amino group to alpha-ketoglutarate, forming pyruvate and glutamate, which can then be used in gluconeogenesis or the citric acid cycle.
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Fate of amino acid carbon skeletons
After deamination, carbon skeletons of amino acids enter various pathways: pyruvate for glucogenic, acetyl-CoA for ketogenic, or directly into the citric acid cycle.
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One-carbon pool
A set of single-carbon units carried by tetrahydrofolate or S-adenosylmethionine, derived from amino acids like serine, and used for syntheses like purine bases.
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Serine metabolism
Serine is synthesized from 3-phosphoglycerate and can donate a one-carbon unit to the tetrahydrofolate pool, also serving as a precursor for glycine and cysteine.
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Glycine metabolism
Glycine is used in heme synthesis, as a neurotransmitter, and can be converted to serine, with its catabolism producing CO2 and ammonia.
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Asparagine synthesis
Asparagine is formed by the amidation of aspartate using glutamine as the ammonia donor, making it non-essential under normal conditions.
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Glutamine role in metabolism
Glutamine transports ammonia to the liver for the urea cycle and serves as a fuel for rapidly dividing cells, like those in the gut and immune system.
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Transamination reaction equation
The general equation is: an amino acid + alpha-keto acid ⇌ another alpha-keto acid + another amino acid, catalyzed by pyridoxal phosphate-dependent enzymes.
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Urea cycle equation
Overall, two ammonia molecules and one CO2 combine to form urea and water, with the net reaction incorporating nitrogen from amino acid breakdown.
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Inborn errors of metabolism
Genetic defects in enzymes of amino acid pathways, such as in PKU, leading to accumulation of toxic intermediates and requiring dietary management.